1. Field of the Invention
The present invention relates to the field of energy conversion for lighting generally and more particularly to an electronic ballast suitable for use with gaseous-discharge lamps or other non-linear or negative-resistance loads.
2. Description of the Related Art
Gaseous-discharge lamps, i.e. lamps in which light is generated when an electric current or discharge is passed through a gaseous medium, are well known to the lighting field. Fluoresecent-type gaseous-discharge lamps have been employed for years to provide relatively efficient indoor lighting, such as for office buildings.
Unlike incandescent lamps, which are self-current limiting as a result of their positive-resistance characteristics, gaseous-discharge lamps have a negative-resistance characteristic. For this reason, gaseous-discharge lamps are operated in conjunction with a ballast, which provides the requisite current limiting. Traditionally, ballasts are of core and coil construction. One form is that of a simple choke which provides an inductive impedance for current limiting. Another form is that of a transformer. The transformer form permits voltage conditioning, such as providing a high break-down potential, which is required for starting instant-start-type fluorescent lamps by ionizing to a plasma the gas therein. For rapid-start-type fluorescent lamps, the transformer includes a pair of windings for energizing the lamp filaments and separated by a high-voltage winding having a high reactance for current limiting. Additionally, a magnetic shunt may be included in the transformer to limit the energy transferred through the magnetic path.
Unfortunately, traditional core-and-coil-type ballasts are relatively inefficient due to substantial heat generated losses that are generally equally divided between copper losses in the coil and core losses in the relatively inexpensive grades of iron employed therein. For example, a traditional core-and-coil-type ballast employed in a dual forty watt lamp fixture may dissipate from ten to fifteen watts, causing the ballast to run hot. Further, in many applications, such as in office buildings, this ballast-generated heat must be removed by air conditioning equipment, which is itself relatively inefficient. Another problem is that core-and-coil-type ballasts are relatively heavy in weight and thereby requiring that associated fixtures be substantial in size and strength.
The regulation afforded by traditional core-and-coil-type ballasts is also relatively poor. Typically, the operating level of fluorescent fixtures employing such ballasts varies directly with the power-line voltage. Thus, in many applications, excessive lighting is often employed to insure that minimum lighting levels are achieved resulting in dissipation of excessive power.
Among other problems associated with gaseous-discharge lamps is that they are less efficient when operated at the normal sixty Hz line frequency than when operated at higher frequencies. Fluorescent lamps are often difficult to start when cold and consequently flicker for some time until they are sufficiently warm. Fluorescent lamps require core-and-coil-ballast phasing both to reduce stroboscopic effects and to increase the power factor such lamps present to the AC power line via the ballast.
Also, core-and-coil-type ballasts provide little isolation from the AC power line. Consequently, such ballast may pose a safety hazard due to potential electrocution to persons who come in contact with them.
These problems are overcome by the "Single-Ended Ballast Circuit", which is disclosed in U.S. Pat. No. 5,028,846. Briefly, the above-mentioned circuit employs a DC power supply, a free-running oscillator, a transistor (switch), and a current-limiting (ballasting) network, all configured to generate a high-frequency AC power source on a line. Specifically, the transistor is configured as a switch responsive to a signal generated by the oscillator and operative to periodically couple the line to a circuit common. For this purpose, the transistor gate is connected to the output of the oscillator, the transistor drain is connected to the line, and the transistor source is connected to the circuit common. The current-limiting (ballasting) network is shown (in FIG. 3 of the above-mentioned patent) to include a first capacitor, which is connected between the line and the circuit common. In addition, the network includes a transformer (inductive means) having a primary winding connected between a DC power supply potential and the line and a secondary winding. The network further includes an inductor and two other capacitors connected in series across the transformer secondary winding. Two fluorescent lamps are connected in series across one of the capacitors. The network is operative to provide an impedance transformation to couple the fluorescent lamps to the high-frequency AC power source developed on the line. The network develops the desired open circuit output voltage for starting the fluorescent lamps. In addition, the network provides the desired source impedance as seen by the lamps. Also, the network establishes the operating Q for the desired output waveform and the desired load impedance and phase angle, as seen by the transistor, for both the operating and open circuit conditions. Finally, the network is operative to provide an inductive power supply feed for the transistor.
Unfortunately, in the above-mentioned ballast circuit (and most other so-called "single ended" designs using one power transistor switch), there are many disadvantages which limit the applications or have cost disadvantages. Specifically, the disadvantages for the circuit of U.S. Pat. No. 5,028,846 are as follows:
1. The voltage Waveform across the switch transistor (and the first network capacitor) is a continuous series of (nearly) half-sine pulses which creates a voltage stress during the "off tile" of the switch at least 3.2 times the instantaneous voltage of the DC supply voltage. This limits this circuit to AC supply voltages of 120 volts or less, and/or requires the use of an expensive transistor;
2. Since the first network capacitor is directly across the switch transistor, it also must be rated to handle the maximum peak voltage and the current stress due to the waveforms shape;
3. For maximum efficiency (very important for electronic ballasts) zero-voltage or zero-current switching for the power transistor is necessary. This circuit can have zero-voltage switching on the trailing edge of the half-sine pulse only if the voltage falls to zero before the switch is turned on.
The exact shape of these half-sine pulses on the trailing edge is determined by the network component values, including the load impedance. The actual lamp impedance is sensitive to temperature and the circuit sensitivity to load impedance is quite high. This means that, if zero-switching is to be assured, the network values must assure slight "under-damping" for the worst-case load impedance. Thus, the circuit can be very much under-damped for other than worst-case load conditions. More under-damping means even higher switch voltage stress (4 or more times the DC supply). This dilemma means that a trade-off must be made between ballast efficiency and transistor cost;
4. These half-sine pulses at the input to the network also create inefficiency in the network itself. The harmonics due to the half-sine shape increase the losses primarily in the two magnetic components, further reducing ballast efficiency or increasing cost;
5. The half-size pulse waveshape can also cause problems witch the lamp current waveshape, unless the network has sufficiently high "Q". Unfortunately, high Q means either lower efficiency or higher cost;
6. The actual lamp impedance also depends on the lamp current. Unless a low-ripple (more expensive) DC supply is used, this lamp characteristic will create additional load variations during the AC line cycle that need to be accounted for in the worst-case design dilemma described in 3., above.